The current art teaches three known methods of transporting natural gas across bodies of water. A first method is by way of subsea pipeline. A second method is by way of ship transport as liquefied natural gas (LNG). A third method is by way of barge, or above deck on a ship, as compressed natural gas (CNG). Each method has its inherent advantages and disadvantages.
Subsea pipeline technology is well known for water depths of less than 1000 feet. The cost of deep water subsea pipelines is very high and methods of repairing and maintaining deep water subsea pipelines are just being pioneered. Transport by subsea pipeline is often not a viable option when crossing bodies of water exceeding 1000 feet in depth. A further disadvantage of subsea pipelines is that, once laid, it is impractical to relocate.
Liquefied natural gas systems, or LNG systems, require natural gas to be liquefied. This process greatly increases the fuel's density, thereby allowing relatively few numbers of ships to transport large volumes of natural gas over long distances. An LNG system requires a large investment for liquefaction facilities at the shipping point and for re-gasification facilities at the delivery point. In many cases, the capital cost of constructing LNG facilities is too high to make LNG a viable option. In other instances, the political risk at the delivery and/or supply point may make expensive LNG facilities unacceptable. A further disadvantage of LNG is that even on short routes, where only one or two LNG ships are required, the transportation economics are still burdened by the high cost of full shore facilities. The shortcoming of a LNG transport system is the high cost of the shore facilities which, on short distance routes, becomes an overwhelming portion of the capital cost.
Natural gas prices are currently increasing rapidly due to an inability to meet demand. Unfortunately, the LNG import terminals existing in the United States are presently operating at capacity. New import terminals of the type currently used in the United States cost hundreds of millions of dollars to build. Moreover, it is very difficult and expensive to find and acquire permissible sites for such facilities. Besides the space needed for the import tanks, pumps, vaporizers, etc., large impoundment safety areas must also be provided around all above-ground LNG storage and handling vessels and equipment. LNG import facilities also consume large amounts of fuel, gas and/or electrical energy for pumping the LNG from storage and vaporizing the material for delivery to gas distribution systems.
Compressed natural gas, or CNG, can be transported by way of barge or above deck on a ship. For the method to work, the CNG is cooled to a temperature around −75 degrees Fahrenheit at a pressure of around 1150 psi. The CNG is placed into pressure vessels contained within an insulated cargo hold of a ship. Cargo refrigeration facilities are not usually provided aboard the ship. A disadvantage of this system is the requirement for connecting and disconnecting the barges into the shuttles that takes time and reduces efficiency. Further disadvantages include the limited seaworthiness of the multi-barge shuttles and the complicated mating systems that adversely affect reliability and increase costs. In addition, barge systems are unreliable in heavy seas. Finally, current CNG systems have the problem of dealing with the inevitable expansion of gas in a safe manner as the gas warmed during transport.
The amount of equipment and the complexity of the inter-connection of the manifolding and valving system in the barge gas transportation system bears a direct relation to the number of individual cylinders carried onboard the barge. Accordingly, a significant expense is associated with the manifolding and valving connecting the gas cylinders. Thus, the need has arisen to find a storage system for compressed gas that can both contain larger quantities of compressed gas and to simplify the system of complex manifolds and valves.
Current systems place their cargo of natural gas compressed is the hold of floating vessels. The hold is sealed off as a closed space and then the entire hold is refrigerated. As the refrigerated gas warms during the trip, the released vapor gas tends to collect in the hold causing a dangerous explosion situation. To counter this problem, current system use nitrogen gas systems to displace the vapor gas with inert nitrogen gas. The nitrogen gas system, however, is very expensive and requires increased costs in order to maintain working order. In addition, the nitrogen gas system uses valuable space on the floating vessel that can be used to hold more cargo.
A need exists to transfer compressed natural gas safely across heavy seas to locations greater than 500 nautical miles.
A need exists for a system that can solve the concerns of the inevitable expansion of gas experienced as CNG warms during transport.